1. Hello, world in MPI #include <stdio.h> #include "mpi.h"
int main( int argc, char *argv[]) {
int rank, size;
MPI_Init( &argc, &argv );
MPI_Comm_size( MPI_COMM_WORLD, &size );
MPI_Comm_rank( MPI_COMM_WORLD, &rank );
printf( "Hello world from process %d of %d\n", rank, size );
MPI_Finalize();
return 0; }

2. MPI in nine routines (all you really need)
MPI_Init Initialize
MPI_Finalize Finalize
MPI_Comm_size How many processes?
MPI_Comm_rank Which process am I?
MPI_Wtime Timer
MPI_Send Send data to one proc
MPI_Recv Receive data from one proc
MPI_Bcast Broadcast data to all procs
MPI_Reduce Combine data from all procs

3. Ten more MPI routines (sometimes useful)
More group routines (like Bcast and Reduce):
MPI_Alltoall, MPI_Alltoallv
MPI_Scatter, MPI_Gather
Non-blocking send and receive:
MPI_Isend, MPI_Irecv
MPI_Wait, MPI_Test, MPI_Probe, MPI_Iprobe
Synchronization:
MPI_Barrier

4. Example: Send an integer x from proc 0 to proc 1
MPI_Comm_rank(MPI_COMM_WORLD,&myrank); /* get rank */
int msgtag = 1;
if (myrank == 0) {
int x = 17;
MPI_Send(&x, 1, MPI_INT, 1, msgtag, MPI_COMM_WORLD);
} else if (myrank == 1) {
int x;
MPI_Recv(&x, 1, MPI_INT,0,msgtag,MPI_COMM_WORLD,&status); }

5. Some MPI Concepts Communicator
A set of processes that are allowed to communicate among themselves.
Kind of like a “radio channel”.
Default communicator: MPI_COMM_WORLD
A library can use its own communicator, separated from that of a user program.

6. Some MPI Concepts Data Type What kind of data is being sent/recvd?
Mostly just names for C data types
MPI_INT, MPI_CHAR, MPI_DOUBLE, etc.

7. Some MPI Concepts Message Tag Arbitrary (integer) label for a message
Tag of Send must match tag of Recv
Useful for error checking & debugging

8. Parameters of blocking send
MPI_Send(buf, count, datatype, dest, tag, comm)
Address of
Datatype of
Message tag
send b
uff er
each item
Number of items
Rank of destination
Comm
unicator
to send
process

9. Parameters of blocking receive
MPI_Recv(buf, count, datatype, src, tag, comm, status)
Status
Address of
Datatype of
Message tag
after oper
ation
receiv
e b
uff er
each item
Maxim
um n
umber
Rank of source
Comm
unicator
of items to receiv e
process

10. Example: Send an integer x from proc 0 to proc 1
MPI_Comm_rank(MPI_COMM_WORLD,&myrank); /* get rank */
int msgtag = 1;
if (myrank == 0) {
int x;
MPI_Send(&x, 1, MPI_INT, 1, msgtag, MPI_COMM_WORLD);
} else if (myrank == 1) {
MPI_Recv(&x, 1, MPI_INT,0,msgtag,MPI_COMM_WORLD,&status); }